Fusion Science and Technology / Volume 50 / Number 3 / October 2006 / Pages 352-360
Technical Paper / Stellarators
In the helical-axis heliotron configuration, bumpiness of the Fourier components in Boozer coordinates is introduced to control the neoclassical transport. The bumpiness helps not only to align the mod-Bmin contours with the magnetic flux surfaces but also to control the balance of bootstrap currents due to helical and toroidal ripples. Effects of bumpiness control on the plasma performance (noninductive currents, fast-ion behavior, and global energy confinement) have been investigated in Heliotron J by selecting three configurations with different bumpiness ([curly epsilon]b = B04/B00 = 0.01, 0.06, and 0.15 at = 2/3) but almost the same edge rotational transform and plasma volume. The dependence of noninductive toroidal currents is qualitatively consistent with the neoclassical prediction for the bootstrap current. The high-bumpiness configuration seems to be preferable for the confinement of fast ions. However, the longer global energy confinement time is not observed in the highest-bumpiness configuration ([curly epsilon]b = 0.15). When the dependence of the effective ripple modulation amplitude in International Stellarator Scaling 04 scaling is examined, the experimental results show that the normalized global energy confinement time seems long in the configuration with the minimum effective ripple modulation amplitude, where [curly epsilon]b is 0.06.